The present invention generally relates to ceramic/metal materials and, more specifically, to carbide- and boride-based materials and methods for making these materials.
Composite materials that have a metal matrix and a strengthening or reinforcing phase such as ceramic particulates, whiskers, fibers, or the like, are used for a variety of applications because they combine some of the stiffness and wear resistance of the reinforcing phase with the ductility and toughness of the metal matrix. These materials are generally referred to as metal matrix composites (MMCs). Such a composite generally shows improved strength, wear resistance, and high temperature compatibility relative to using the metal only and may also weigh less than the same article made from metal alone.
Likewise, composites in which the bonding or matrix phase is made from a ceramic material and the reinforcing phase is a metal or other ceramic are also used for manufacture of articles. These materials are generally referred to as ceramic matrix composites (CMCs). These materials also reflect improved strength, stiffness, wear resistance, and temperature compatibility compared to using the metal alone and also show improved ductility and toughness when compared to the ceramic component alone. While ceramic materials alone provide improved temperature resistance and strength over metals alone, a primary disadvantage in the use of ceramic materials alone is lack of reliability. This low reliability stems from low fracture toughness and brittle fracture behavior, which renders ceramics sensitive to rapid catastrophic failure in response to accidental overloading, contact damage, or rapid temperature changes. Addition of a metal reinforcing phase provides improved reliability over ceramics alone.
Because of the improved material properties realized by combining the metal and ceramic into a composite material, these materials have found use in a variety of applications, including applications in the aerospace, automotive, medical, and sports industries. One way in which to create such a composite material is vapor-phase oxidation of a bulk molten metal, usually in an inert graphite or alumina crucible, by a gas to produce a solid ceramic-containing body via a directed growth process, such as described in U.S. Pat. No. 4,713,360. A reaction product will form initially on the exposed surface of a pool of the molten metal and then grow outward, fed by transport of additional metal through channels in the ceramic product of the oxidation reaction between the parent metal and the gas phase oxidant to further react with the gas.
A direct metal oxide process to create a composite is a unidirectional process and growth will occur from one side of the material to the other. Thus, the material may not be homogenous from one side to the other. It is also known that infiltration of porous ceramic materials (e.g., Al2 O3, B4C, SiC) with molten metal can result in a ceramic/metal composite.
Another method of forming a ceramic/metal composite is by non-vapor phase oxidation of the molten metal by a sacrificial ceramic preform, such as described in U.S. Pat. No. 5,214,011, which includes placement of the sacrificial preform in contact with a molten metal at a temperature greater than the melting point of the metal but less than the melting point or softening point of the sacrificial preform. The sacrificial preform and the molten metal are maintained in contact at the elevated temperature for a time sufficient to allow the sacrificial preform to at least partially transform into a ceramic metal oxide body containing a metallic phase.
Some composite materials also include whiskers, particles, or other additions, such as ceramic carbide particles, to improve specific properties. Most structural materials that contain carbide ceramic particles use small carbide particles because small particles sinter better and result in improved properties. In MMC materials, small particles are used because larger particles would reduce the strength and fracture toughness. In MMC materials, these small particles are easy to dislodge, because they are held by a soft/ductile metal matrix. Further, because their diameter is small, only a small area of interfacial bonding is holding them in place.